This is a proposal for the continued refinement of the thermal diffusion probe technique and instrument for continuous quantification of perfusion in small volumes of tissue. An isolated gracilis muscle preparation will be used to establish the validity of the tissue perfusion model, define the effects of the thermal diffusion probe on tissue and quantify the influence of blood vessels of varying size, flow, and distance from the probe on blood flow measured by the probe. Values of kappa of blood of varying hematocrit will be established and the effect of initial tissue temperature gradients on the accuracy of perfusion measurements will be established. A dynamic technique for the determination of intrinsic tissue thermal conductivity in the presence of flow using two probes of different sizes will be developed. The efficacy of using effective thermal diffusivity measurements as a second measure of perfusion will also be established. Software development will be required in order to provide simultaneous transient and continuous mode operation, improved precision and to support new test situations.